(1) Field of the Invention
The present invention relates to a photocurrent amplifier circuit and an optical pick-up device in which a photocurrent amplifier circuit is used, and particularly to a technique which realizes a simple and downsized photocurrent amplifier circuit.
(2) Description of the Related Art
Optical disc media such as Compact discs (CDs) and Digital Versatile discs (DVDs) have been widely used for recording is digital information represented by video and audio which requires a large capacity. In order to read and/or write information from and/or on these various types of optical disc media (hereinafter simply referred to as media), as commonly known, laser lights with different wavelengths are used in accordance with the types of such media.
A conventional downsized optical pick-up device which is compliant with any of CDs and DVDs typically includes a two-wavelength semiconductor laser device which is used as a light source and a single optical system which is used in common for both the wavelengths of the laser lights. Subsequently, through the optical system, the respective lasers are projected onto points, on the medium, which are different for each of the wavelengths which are dependent on the distance of the emitting points of the lasers having the respective wavelengths. By performing photo-electric conversion of the light reflected from the medium using light receiving devices which are set for the respective wavelengths, an electric signal is obtained, amplified and outputted.
A well-known light receiving amplifier device which is suitable for an optical pick-up device like this is a light receiving amplifier device configured by differential amplifiers which are set for individual light receiving devices with different wavelengths and an output amplifier which selectively amplifies one of the outputs (for example, refer to FIG. 3 and FIG. 4 of Patent Reference 1: Japanese Laid-Open Patent No. 2004-22051).
However, with a conventional light receiving amplifier device, there is a problem that it is difficult to downsize a circuit because differential amplifiers are set for individual light receiving devices with different wavelengths.
This problem is serious especially in the case of wishing to further downsize an optical pick-up device with present functions or wishing to realize a simple and downsized three-wavelength is optical pick-up device which is compliant with not only CDs and DVDs but also Blu-ray Discs (BDs) with a memory capacity bigger than those of CDs and DVDs.
The present invention has been conceived considering the above-described problem, and aims to provide a photocurrent amplifier circuit which selectively amplifies one or more of the photocurrents obtained from a plurality of light receiving devices, and can be realized to be simple and downsized.